1. Field of the Invention
The present invention relates generally to electro-optical inspection systems, and more particularly to an automated photomask inspection apparatus for detecting defects on optical masks and reticles and the like.
2. Brief Description of the Prior Art
Integrated circuits are made by photolithographic processes which use photomasks or reticles and an associated light source to project a circuit image onto a silicon wafer. A high production yield is contingent on having defectless masks and reticles. Since it is inevitable that defects will occur in the mask, these defects have to be found and repaired prior to using the mask.
Automated mask inspection systems have existed for over 15 years. The earliest such system, the Bell Telephone Laboratories AMIS system (John Bruning et al., "An Automated Mask Inspection System--AMIS", IEEE Transactions on Electron Devices, Vol. ED-22, No. 7 July 1971, pp 487 to 495), used a laser that scanned the mask. Subsequent systems used a linear sensor to inspect an image projected by the mask, such as described by Levy et al. (U.S. Pat. No. 4,247,203, "Automatic Photomask Inspection System and Apparatus") who teach die-to-die inspection, i.e., inspection of two adjacent dice by comparing them to each other. Alternately, Danielson et al. (U.S. Pat. No. 4,926,489, "Reticle Inspection System") teach die-to-database inspection, i.e. inspection of the reticle by comparison to the database from which the reticle was made.
As the complexity of the integrated circuits has increased, so has the demand on the inspection process. Both the need for resolving smaller defects and for inspecting larger areas have resulted in much greater speed requirements, in terms of number of picture elements per second processed. The increased demands have given rise to improvements described in a number of subsequently issued patents, such as U.S. Pat. Nos. 4,247,203, entitled "Automatic Photomask Inspection System and Apparatus", Levy et al., issued Jan. 27, 1981; 4,579,455, entitled "Photomask Inspection Apparatus and Method with Improved Defect Detection", Levy et al., issued Apr. 1, 1986; 4,633,504, entitled "Automatic Photomask Inspection System Having Image Enhancement Means", Mark J. Wihl, issued Dec. 30, 1986; and 4,805,123, entitled "Automatic Photomask Inspection and Reticle Inspection Method and Apparatus Including Improved Defect Detector and Alignment Subsystem", Specht et al., issued Feb. 14, 1989. Also of relevance is some prior art in the wafer inspection area, such as U.S. Pat. No. 4,644,172, entitled "Electronic Control of an Automatic Wafer Inspection System", Sandland et al., issued Feb. 17, 1987.
Another force driving the development of improved inspection techniques is the emergence of phase shift mask technology. With this technology it will be possible to print finer linewidths, down to 0.25 micrometers or less. This technology is described by Burn J. Lin, "Phase-Shifting and Other Challenges in Optical Mask Technology", Proceedings of the 10th Annual Symposium on Microlithography. SPIE,--the International Society of Optical Engineering, Vol. 1496, pages 54 to 79.
The above improvements teach the automatic detection of defects on conventional optical masks and reticles. In all of these systems, conventional lighting is used and the images are captured by linear array sensors. These two system choices limit the signal-to-noise ratio and hence the speed of inspection.